[llvm] [DAG] Refactor X86 combineVSelectWithAllOnesOrZeros fold into a generic DAG Combine (PR #145298)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> I think you might need to check getBooleanContents == ZeroOrNegativeOneBooleanContent for a number of these combines to be valid?

Do you think there are cases that the ComputeNumSignBits check doesn't cover?

https://github.com/llvm/llvm-project/pull/145298